You can do that in Microsoft Word or if you don't have Word you can use the program that comes with most computers....I think it's called Notepad? Once you get your letter written, copy and paste it into the letter section. The only downside is monitoring how much space you have left, but after a few tries you should be able to get the hang of how much room you're allowed.

We pretty much do what Marci Ficht recommends here, using Libre Office because we do not have Microsoft Word at home. We type up our letters, modifying as we go along. Get them into final form. And then copy and paste into the online form when ready. IF I need to gather a bunch of attachments, I will make a folder for the letter and put the attachments in there as well, converting them to JPG form when needed.

All of you have far better technology then I do and are way more tech savvy than I am. I'm working on a $14.00 almost smart phone, and a pretty dumb person, internet access at the house is more and more limited every day, so I have to do all online stuff at work, on the sly,
since the factory big wigs seem to frown on using cell phone during working hours for some odd reason
(shrugs)